U.S. patent number 9,029,974 [Application Number 14/023,819] was granted by the patent office on 2015-05-12 for semiconductor device, junction field effect transistor and vertical field effect transistor.
This patent grant is currently assigned to Infineon Technologies AG. The grantee listed for this patent is Infineon Technologies AG. Invention is credited to Romain Esteve, Jens Konrath, Daniel Kueck, David Laforet, Cedric Ouvrard, Roland Rupp, Andreas Voerckel, Wolfgang Werner.
United States Patent |
9,029,974 |
Esteve , et al. |
May 12, 2015 |
Semiconductor device, junction field effect transistor and vertical
field effect transistor
Abstract
A semiconductor device according to an embodiment is at least
partially arranged in or on a substrate and includes a recess
forming a mesa, wherein the mesa extends along a direction into the
substrate to a bottom plane of the recess and includes a
semiconducting material of a first conductivity type, the
semiconducting material of the mesa including at least locally a
first doping concentration not extending further into the substrate
than the bottom plane. The semiconductor device further includes an
electrically conductive structure arranged at least partially along
a sidewall of the mesa, the electrically conductive structure
forming a Schottky or Schottky-like electrical contact with the
semiconducting material of the mesa, wherein the substrate
comprises the semiconducting material of the first conductivity
type comprising at least locally a second doping concentration
different from the first doping concentration along a projection of
the mesa into the substrate.
Inventors: |
Esteve; Romain (Treffen am
Ossiacher See, AT), Konrath; Jens (Villach,
AT), Kueck; Daniel (Villach, AT), Laforet;
David (Villach, AT), Ouvrard; Cedric (Villach,
AT), Rupp; Roland (Lauf, DE), Voerckel;
Andreas (Villach, AT), Werner; Wolfgang (Munich,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Infineon Technologies AG |
Neubiberg |
N/A |
DE |
|
|
Assignee: |
Infineon Technologies AG
(Neubiberg, DE)
|
Family
ID: |
52478719 |
Appl.
No.: |
14/023,819 |
Filed: |
September 11, 2013 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20150069411 A1 |
Mar 12, 2015 |
|
Current U.S.
Class: |
257/471 |
Current CPC
Class: |
H01L
29/7813 (20130101); H01L 29/06 (20130101); H01L
29/812 (20130101); H01L 29/0623 (20130101); H01L
29/7827 (20130101); H01L 29/872 (20130101); H01L
29/7806 (20130101); H01L 29/8083 (20130101); H01L
29/0657 (20130101); H01L 29/36 (20130101); H01L
29/1608 (20130101); H01L 29/1095 (20130101) |
Current International
Class: |
H01L
29/47 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10161139 |
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Jul 2004 |
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DE |
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102008047998 |
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Apr 2009 |
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DE |
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0122498 |
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Mar 2001 |
|
WO |
|
Other References
Okumura, K., et al., "Ultra Low On-Resistance SiC Trench Devices,"
Power Electronics Europe, Jun. 21, 2012, pp. 22-25, Issue 4. cited
by applicant.
|
Primary Examiner: Sandvik; Benjamin
Assistant Examiner: Khan; Farid
Attorney, Agent or Firm: Murphy, Bilak & Homiller,
PLLC
Claims
What is claimed is:
1. A semiconductor device at least partially arranged in or on a
substrate, the semiconductor device comprising: a recess forming a
mesa, the mesa extending along a direction into the substrate to a
bottom plane of the recess, the mesa comprising a semiconducting
material of a first conductivity type, the semiconducting material
of the mesa comprising at least locally a first doping
concentration not extending further into the substrate than the
bottom plane; a doped region of a second conductivity type arranged
at least partially adjacent to the bottom plane of the recess along
a projection of the recess into the substrate such that the mesa is
accessible by charge carriers avoiding the doped region; a further
doped region of the first conductivity type arranged along a
projection of the mesa into the substrate and in a direction
perpendicular to the projection adjacent to the doped region; and
an electrically conductive structure arranged at least partially
along a sidewall of the mesa, the electrically conductive structure
forming a Schottky or Schottky-like electrical contact with the
semiconducting material of the mesa, wherein the substrate
comprises the semiconducting material of the first conductivity
type comprising at least locally a second doping concentration
different from the first doping concentration along a projection of
the mesa into the substrate.
2. The semiconductor device according to claim 1, wherein the first
doping concentration is higher than the second doping
concentration.
3. The semiconductor device according to claim 1, wherein the
semiconductor material comprising the second doping concentration
is part of a drift area for charge carriers.
4. The semiconductor device according to claim 1, wherein the
semiconductor material comprising the second doping concentration
is arranged adjacent to the semiconductor material of the mesa with
the first doping concentration.
5. The semiconductor device according to claim 1, wherein the doped
region extends into a projection of the mesa into the
substrate.
6. The semiconductor device according to claim 1, wherein the
further doped region comprises a third doping concentration, the
third doping concentration being larger than the first doping
concentration and the second doping concentration.
7. The semiconductor device according to claim 1, wherein the
further doped region extends further into the substrate than the
doped region of the second conductivity type and below the doped
region of the second conductivity type.
8. The semiconductor device according to claim 1, wherein the
recess comprises an electrical contact structure arranged on the
bottom plane of the recess and configured to electrically couple
the doped region of the second conductivity type to the
electrically conductive structure.
9. The semiconductor device according to claim 1, wherein the
recess comprises an electrically insulating structure arranged
partially along the sidewall of the recess at the bottom plane.
10. The semiconductor device according to claim 1, wherein the mesa
comprises a top surface, wherein the electrically conductive
structure is further arranged on top of the top surface of the mesa
forming an upper part of the Schottky or Schottky-like electrical
contact with the semiconducting material of the mesa, and wherein
the electrically conductive structure arranged on the sidewall of
the mesa forms a lower part of the Schottky or Schottky-like
electrical contact.
11. The semiconductor device according to claim 10, wherein the
Schottky or Schottky-like electrical contact is configured to
comprise a diode-like characteristic with a threshold voltage in a
forward-biased state, wherein the upper part of the Schottky or
Schottky-like electric contact comprises a diode-like
characteristic with a lower threshold voltage than the lower part
of the Schottky or Schottky-like electric contact.
12. The semiconductor device according to claim 1, wherein the mesa
comprises a height along a direction into the substrate and a width
perpendicular to the direction into the substrate, and wherein the
height is at least equal to the width.
13. The semiconductor device according to claim 1, wherein the
semiconductor device is configured such that in a reverse-biased
state of the Schottky or Schottky-like electric contact an electric
field strength along the sidewall of the mesa is essentially
constant along a portion of the sidewall comprising at least 50% a
height along a direction into the substrate of the sidewall.
14. The semiconductor device according to claim 1, wherein the
Schottky or Schottky-like electrical contact comprises a
characteristic reverse electric field strength in a reverse-biased
state, wherein the semiconductor device is configured to cause a
depletion of charge carriers in an area along a projection of the
mesa into the substrate such that, when a pinch-off voltage in the
range of 5 V to 50 V is applied to the electrically conductive
structure and a counter electrode such that the Schottky or
Schottky-like electrical contact is in the reverse-biased state,
the characteristic reverse electric field strength at the Schottky
or Schottky-like electrical contact is not exceeded.
15. The semiconductor device according to claim 1, wherein the
semiconducting material is a Silicon Carbide (SiC).
16. The semiconductor device according to claim 15, wherein the
sidewall is parallel to at least one of a (112;.sup.- 0)-plane and
a (11;.sup.- 00)-plane.
17. The semiconductor device according to claim 1, wherein an
electric field at the sidewall at a normal operating voltage is at
most 30% of a maximum electric field below the bottom plane.
Description
FIELD
Embodiments relate to a semiconductor device, a junction field
effect transistor (JFET) and a vertical field effect transistor
(vertical FET).
BACKGROUND
In many semiconductor devices, diode-like structures are used for
different purposes, for instance, to protect an active region of a
semiconductor device. In terms of their current-voltage
characteristics, diode-like structures typically comprise a
forward-biased threshold voltage and a characteristic reverse
voltage in a reverse-biased state, above which a leakage current
starts to increase significantly.
Typically, a tendency exists to reduce the threshold voltage in the
forward-biased state, while the characteristic reverse voltage in
the reverse-biased state is to be raised. Naturally, further
boundary conditions including process control of the manufacturing
process, available space on a substrate of the semiconductor device
and other technical restrictions may pose further boundary
conditions onto the layout of a semiconductor device and its
fabrication process.
SUMMARY
Therefore, a demand exists to improve a trade-off between a
performance of a semiconductor device and fabrication-related
boundary conditions.
A semiconductor device according to an embodiment is at least
partially arranged in or on a substrate. The semiconductor device
comprises a recess forming a mesa, such that the mesa extends along
a direction into the substrate to a bottom plane of the recess. The
mesa comprises a semiconducting material of a first conductivity
type, wherein the semiconducting material of the mesa comprises at
least locally a first doping concentration not extending further
into the substrate than the bottom plane. The semiconductor device
further comprises an electrically-conductive structure arranged at
least partially along a sidewall of the mesa. The
electrically-conductive structure forms a Schottky or Schottky-like
electrical contact with a semiconducting material of the mesa,
wherein the substrate comprises the semiconducting material of the
first conductivity type comprising at least locally a second doping
concentration different from the first doping concentration along a
projection of the mesa into the substrate.
A junction field effect transistor (JFET) according to an
embodiment is arranged at least partially in or on a substrate. The
JFET comprises a recess forming a mesa, the mesa extending along a
direction into the substrate to a bottom plane of the recess. The
mesa comprises a semiconducting material of a first conductivity
type. The JFET further comprises an electrically-conductive
structure arranged at least partially along a sidewall of the mesa,
wherein the electrically-conductive structure forms a Schottky or
Schottky-like electrical contact with the semiconducting material
of the mesa. The JFET further comprises a doped region of a second
conductivity type arranged at least partially adjacent to the
bottom plane of the recess along a projection into the substrate
such that the mesa is accessible by charge carriers avoiding the
doped region. The JFET further comprises a drain contact and a
drift area, wherein the drift area comprises the semiconducting
material of the first conductivity type. The drift area is arranged
along the direction into the substrate between the drain contact
and the electrically-conductive structure. The JFET further
comprises a source region electrically coupled to the
electrically-conductive structure and comprises the semiconducting
material of the first conductivity type. The source region is shut
off from the drift area by a semiconductor region of the second
conductivity type at least partially formed by the doped region.
Furthermore, the JFET comprises a gate stack arrangement comprising
a first layer of a first conductivity type, a second layer of the
second conductivity type and a gate contact. The second layer is
arranged between the first layer and the gate contact. The first
layer is in contact with a source region and the semiconductor
region of the second conductivity type.
A vertical field effect transistor (FET) according to an embodiment
is arranged at least partially in or on a substrate. The vertical
FET comprises a recess forming a mesa, wherein the mesa extends
along a direction into the substrate to a bottom plane of the
recess. The mesa comprises a semiconducting material of a first
conductivity type. The vertical FET further comprises an
electrically-conductive structure arranged at least partially along
a sidewall of the mesa. The electrically-conductive structure forms
a Schottky or Schottky-like electrical contact with the
semiconducting material of the mesa. The vertical FET further
comprises a doped region of a second conductivity type arranged at
least partially adjacent to the bottom plane of the recess along a
projection into the substrate such that the mesa is accessible by
charge carriers avoiding the doped region. It further comprises a
drain contact and a drift area, wherein the drift area comprises
the semiconducting material of the first conductivity type. The
drift area is arranged along the direction into the substrate
between the drain contact and the electrically-conductive
structure. The vertical FET further comprises a source region
electrically coupled to the electrically-conductive structure and
comprises the semiconducting material of a first conductivity type.
It further comprises a channel region comprising the semiconducting
material of the second conductivity type and is arranged along the
direction into the substrate between the source region and the
drift area. The vertical FET further comprises a gate contact
arranged in a trench extending into the substrate, wherein the gate
contact is electrically insulated from the source region, the
channel region and the drift area by an insulating film covering at
least partially a sidewall and a bottom of the trench. The source
region is arranged in a direction perpendicular to the direction
into the substrate between the trench and the doped region. The
channel region is at least partially arranged in the direction
perpendicular to the direction into the substrate between the
trench and the doped region.
Embodiments are based on the finding that a trade-off between a
performance of the semiconducting device and a manufacturing
process may be improved by employing a Schottky or Schottky-like
electrical contact being formed at least partially along a sidewall
of the mesa or the recess. The trade-off may further be improved by
using a first doping concentration at least locally for the
semiconducting material comprised in the mesa compared to a second
doping concentration used below the bottom plane of the recess, but
along the projection of the mesa into the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
Several embodiments of the present invention will be described in
the enclosed Figures.
FIG. 1 shows a cross-sectional view of a semiconductor device
according to an embodiment;
FIG. 2 shows a cross-sectional view of a semiconductor device
according to a further embodiment;
FIG. 3 shows a semiconductor device in the form of a SiC-Schottky
diode according to an embodiment;
FIG. 4 shows a cross-sectional view of a semiconductor device
according to an embodiment;
FIG. 5 shows an electrical field distribution of the semiconductor
device shown in FIG. 4;
FIG. 6 shows a cross-sectional view of a junction field effect
transistor (JFET) according to an embodiment; and
FIG. 7 shows a cross-sectional view of a vertical field effect
transistor (FET) according to an embodiment.
DETAILED DESCRIPTION
In the following, embodiments according to the present invention
will be described in more detail. In this context, summarizing
reference signs will be used to describe several objects
simultaneously or to describe common features, dimensions,
characteristics, or the like of these objects. The summarizing
reference signs are based on their individual reference signs.
Moreover, objects appearing in several embodiments or several
figures, but which are identical or at least similar in terms of at
least some of their functions or structural features, will be
denoted with the same or similar reference signs. To avoid
unnecessary repetitions, parts of the description referring to such
objects also relate to the corresponding objects of the different
embodiments or the different figures, unless explicitly or --taking
the context of the description and the figures into
account--implicitly stated otherwise. Therefore, similar or related
objects may be implemented with at least some identical or similar
features, dimensions, and characteristics, but may be also
implemented with differing properties.
Diode-like structures are widely used in semiconductor devices.
They are, for instance, used to protect other active areas of
semiconductor devices, but may also be used as stand-alone electric
elements in circuits which may, optionally, be implemented as
integrated circuits. Naturally, diode-like structures may also be
used in integrated and non-integrated form in more complex circuits
comprising discrete circuit elements and/or integrated
circuits.
Diode-like structures may comprise a current-voltage characteristic
(IVC) which allows a current to flow through the structure in a
forward-biased state, once a threshold voltage is reached or
exceeded. In a reverse-biased state, typically a current flow is
suppressed until a characteristic reverse voltage, is reached or
exceeded. Often, it is desired to obtain low threshold voltages in
the forward-biased state and high characteristic reverse voltage in
the reverse-biased state. In the reverse-biased state at voltages
below the characteristic reverse voltage and/or in the below the
threshold voltage in the forward-biased state, the current may, for
instance, be exponentially suppressed.
Although embodiments may be implemented essentially using any
semiconducting material, in the following a main emphasis is laid
on silicon carbide (SiC) diodes fabricated or formed in or on a
substrate comprising silicon carbide. Correspondingly, the
semiconducting material widely used may also be silicon carbide.
However, it is to be noted that this merely represents an example.
Other embodiments may also be formed based on other semiconducting
materials, such as silicon (Si), III-V semiconductor materials and
II-VI semiconductor materials, to name just a few examples.
Silicon carbide diodes are typically used today in high voltage
applications requiring the semiconductor devices to withstand high
voltages of 100 V or more. Naturally, semiconductor devices based
on silicon carbide may also be used for smaller or even higher
voltages.
Conventionally, silicon carbide Schottky diodes typically have
threshold voltages in the range between 0.8 V and 1.0 V. These
relatively high threshold voltages may cause comparably high static
losses when using and implementing these devices in applications
employing a low power regime. In these applications, the
forward-biased threshold voltage may be a substantial part,
sometimes even the largest part of the forward-biased voltage drop
caused by the respective device. For instance, Schottky diodes may
have a forward voltage drop of approximately 1.4 V at their nominal
current, of which approximately 1.0 V are caused at the Schottky
barrier.
Therefore, a demand exists to reduce the forward-biased threshold
voltage to increase the performance without significantly causing
an increase of the differential resistance, leakage currents or the
like. In the case of a stand-alone three-dimensional silicon
carbide Schottky diode, to reduce the forward-biased threshold
voltage a metal with a reduced Schottky barrier may be used.
Reducing the Schottky barrier may, for instance, be achieved by
choosing a suitable metal, for instance, molybdenum (Mo), tungsten
(W), tantalum (Ta) or hafnium (Hf) instead of titanium (Ti).
Besides the pure metals also conductive metal compounds like metal
nitrides or metal carbides may be used. Additionally or
alternatively, p-doped or n-doped silicon (Si) may also be used.
Moreover, ion implantation techniques may be used to create a
surface near doping region, which may lead to a significant
increase of leakage currents. To reduce the leakage currents, a
relief of an electrical field at the Schottky interface may be
implemented by integrating merged pn-Schottky structures which are
also referred to as MPS. They may for instance comprise a mesa
structure implemented with materials having different Schottky
barriers. For instance, a material with a higher Schottky barrier
may be used at the sidewall, while at a top area of the mesa a
material with a lower Schottky barrier may be used. Introducing the
doped areas may allow to pinch-off the mesa in the reverse-biased
state. Therefore, an electric field strength at the Schottky or
Schottky-like electrical contact may be limited.
The electrically-conductive structure 180 and the semiconducting
material of the mesa 140 may be configured such that both, a
Schottky and a Schottky-like electric contact 185 between the
electrically-conductive structure 180 and the semiconducting
material of the mesa 140 is established, for instance, by
implementing the top region 270 with different material. The term
Schottky-like electrical contact, therefore, also comprises more
traditional Schottky electric contacts.
In the case of a three-dimensional silicon carbide Schottky diode
integrated within a silicon carbide active switch, it may be
desirable to improve a performance of, for instance, a body diode
integrated into silicon carbide active switches, which may be
unfavorable or even unsuitable due to a large band gap, in which
the body diode conduction may be used. This operation is also
referred to as the third quadrant of operation. To improve this, an
external silicon carbide Schottky diode may be implemented, which
may cause a form factor to increase in the case of a non-monolithic
implementation. This may result in a more complex fabrication
process and in increased costs.
FIG. 1 shows a cross-sectional view of a semiconductor device 100
according to an embodiment. The semiconductor device 100 is at
least partially arranged in or on a substrate 110.
The semiconductor 100 comprises a recess 120 in the form of a
trench 130. The recess 120 may be at least partially formed by a
trench 130 in the semiconducting substrate 110. To be a little more
specific, the semiconductor 100 as shown in FIG. 1 comprises a
plurality of--in other words--more than one recesses 120, of which
FIG. 1 shows a first recess 120-1 and a second recess 120-2. The
recesses 120 form one or more mesas 140, which extend along a
direction 150 into the substrate 110 to a bottom plane 160 of the
recesses 120.
The mesa 140 comprises a semiconducting material of a first
conductivity type. The semiconducting material may be, for
instance, silicon carbide (SiC), silicon (Si) or any other
semiconducting material such as III-V semiconducting materials and
II-VI semiconducting materials. The first conductivity type may be,
for instance, an n-type created, for instance, by doping the
respective semiconducting materials accordingly. In this case, a
second conductivity type may be a p-doped semiconducting material.
Naturally, in other embodiments, the role of the first and second
conductivity types may be exchanged with respect to one
another.
The semiconducting material of the mesa 140 comprises at least
locally a first doping concentration N1 which does not extend
further into the substrate 110 along the direction 150 than the
bottom plane 160. The semiconducting material of the mesa 140 may
take on a doping concentration different from the first doping
concentration above the bottom plane 160 or --in other
words--closer to a top region 170 of the mesa 140.
The direction 150, which is also referred to as the vertical
direction or z-direction, is typically arranged perpendicular to a
main surface of the substrate 110. The substrate 110 may, for
instance, be a semiconductor die having, for instance, an
essentially cubic shape with dimensions along a first direction,
which is also referred to as an x-direction, and a second
direction, also referred to as y-direction, being significantly
larger than along the direction 150 perpendicular to both the first
and second directions. Often, the substrate 110 comprises a
thickness along the direction 150 being at least a factor of 5, at
least a factor of 10, at least a factor of 20, at least a factor of
50 or at least a factor of 100 smaller than any of the directions
along the first and second directions. The main surface may, for
instance, be a top surface of the substrate before the processing.
For instance, in the case of a semiconductor device 100 according
to an embodiment, the top regions 170 of the mesas 140 may
optionally be part of the main surface.
The semiconducting device 100 further comprises an
electrically-conductive structure 180-1, 180-2 arranged at least
partially along a sidewall 190 of the mesa 140. The
electrically-conductive structure 180 forms a Schottky or
Schottky-like electrical contact 185 with a semiconducting material
of the mesa 140. The Schottky or Schottky-like electrical contact
185 may comprise a diode-like current-voltage characteristic (IVC)
with a threshold voltage in a forward-biased state and a
characteristic reverse voltage in a reverse-biased state. When a
voltage applied to the electrical contact reaches or surpasses the
threshold voltage in the forward-biased state, the current flowing
through the electrical contact rises significantly, leading to a
significant reduction of a differential resistance, while the
voltages below the threshold voltage in the forward-biased state,
the current flow is essentially, for instance exponentially,
suppressed.
The same is also true in the reverse-biased state for voltages
smaller than the characteristic reverse voltage. For voltages lower
than the characteristic reverse voltage in the reverse-biased
state, the current flowing through the electrical contact is
essentially suppressed, for instance, exponentially suppressed. In
contrast, when the voltage applied reaches or surpasses the
characteristic reverse voltage in the reverse-biased state, the
current rises significantly, for instance, exponentially.
Naturally, instead of voltages the same also applies to electric
fields taking the geometry and further contact-specific parameters
into account. In other words, the IVC may locally be expressed in
terms of a current density and an electrical field applied to the
electric contact such that the IVC is at least partially determined
by a threshold electric field and characteristic reverse electric
field similar to the threshold voltage and the characteristic
reverse voltage, respectively.
In the case of a Schottky or Schottky-like electrical contact a
unipolar charge transport is present across the electrical contact
in the forward-biased state. In other words, in the forward-biased
state a current transport over the electrical contact is dominated
by charge carriers of a single polarity. In the reverse-biased
state below the characteristic reverse voltage or characteristic
reverse electrical field, the charge transport over the electrical
contact is essentially blocked. As a consequence, in the
reverse-biased state, below the characteristic reverse voltage or
characteristic reverse electrical field, a blocking state is
present.
The semiconducting device 100 as shown in FIG. 1 comprises the
semiconducting material of the first conductivity type comprising
at least locally a second doping concentration different from the
first doping concentration along a projection 200 of the mesa 140
into the substrate 110. The projection 200 into the substrate 110
is oriented along the direction 150 from the main surface into the
substrate 110. In other words, the projection 200 is essentially
perpendicular to the first and second directions along which the
substrate 110 has larger dimensions than along the direction
150.
By implementing the higher doping concentration (first doping
concentration) in the mesa 140 above the bottom plane 160 than in
the substrate outside the mesa and, hence, below the bottom plane
160 along the direction 150, the electrical field inside the mesa
can be reduced leading to a more equal field distribution inside
the mesa 140. As a consequence, the electrical contact 185 formed
at least partially along the sidewalls 190 of the mesa 140 or the
recesses 120 can contribute to the current transport more equally.
As a consequence, it may be possible to increase the area used for
the Schottky or Schottky-like electrical contact 185, which may
reduce a voltage drop and, hence, the threshold voltage in the
forward-biased state. In other words, optionally, the first doping
concentration in the mesa 140 may be higher than the second doping
concentration below the bottom plane 160.
The semiconductor material comprising the second doping
concentration below the bottom plane 160 may, for instance, be part
of the drift area 210 of the semiconductor device 100. For
instance, on a back side 220 of the semiconductor device 100, an
electrical contact may be arranged such that an electrical
transport of charge carriers may flow in or against the direction
150 making the semiconductor device 100 a vertical device.
Optionally, the semiconducting material comprising the second
doping concentration may be arranged adjacent to the semiconducting
material of the mesa with the first doping concentration. In the
case that the mesa 140 comprises an essentially homogeneous doping
concentration having the first doping concentration extending down
to the bottom plane 140, the semiconducting material having the
second doping concentration may, for instance, be arranged directly
below the bottom plane. In other words, the semiconducting material
comprising the second doping concentration may also be arranged
adjacent to the bottom plane 160.
Optionally, the semiconducting device 100 may further comprise a
doped region 230-1, 230-2 of the second conductivity type arranged
at least partially adjacent to the bottom plane 160 along a
projection 240 of the recess 120 such that the mesa 140 is
accessible--in both directions--by charge carriers avoiding the
doped region 230. Hence, charge carriers may leave and/or enter the
mesa 140 without entering or leaving the doped regions 230.
In the cross-sectional view of FIG. 1, underneath both recesses
120-1, 120-2 doped regions 230-1, 230-2, respectively, are
arranged. Since the mesa 140 is accessible by charge carriers
without getting into contact with the doped regions 230, a region
exists between two neighboring doped regions 230 comprising the
semiconducting material of the first conductivity type. Due to the
doped regions comprising the semiconducting material of the second
conductivity type, under some operational conditions, a depletion
zone may form at the interface of the doped region 230 and the
semiconducting material of a first conductivity type along the
projection 200 of the mesa 140. Therefore, it may be possible to
pinch off the mesa 140 in the reverse-biased state such that due to
a lack of charge carriers in the depletion zone, such that a major
part of the voltage drops across the depletion zone.
Naturally, in embodiments, the doped region 230 may extend into the
projection 200 of the mesa 140 into the substrate 110. As a
consequence, the depletion zone may form, for instance, in the
reverse-biased state pinching off the mesa 140 more
efficiently.
Optionally, as indicated by dashed lines in FIG. 1, the
semiconducting device 100 may comprise a further doped region 250
of the first conductivity type arranged along the projection 200 of
the mesa 140 into the substrate 110 and in a direction
perpendicular to the projection 200 adjacent to the doped regions
230. Optionally, the further doped region 250 may comprise a third
doping concentration being larger than the first doping
concentration of the mesa 140 and the second doping concentration
along the projection 200 of the mesa 140. This may allow a more
even distribution of the current entering or leaving the mesa 140
in the forward-biased operation and, hence, may enable a more even
distribution of the currents inside the mesa.
The electrically-conductive structure 180 arranged at least
partially along the sidewalls 190 of the recess 120 may be formed
or comprise material or a group of materials. The group of
materials comprises, for instance, metals like aluminum (Al),
titanium (Ti), zinc (Zn), tungsten (W), tantalum, (Ta), molybdenum
(Mo), copper (Cu), nickel (Ni), gold (Au), hafnium (Hf), molybdenum
nitride (MoN), tantalum nitride (Ta.sub.xN.sub.y), titanium nitride
(TiN) and platinum (Pt). However, the group of materials also
comprises alloys as well as doped poly-silicon (poly-Si), undoped
poly-silicon, doped poly-germanium (poly-Ge), undoped
poly-germanium, narrowband semiconducting materials, wideband
semiconducting materials, II-VI semiconducting materials and III-V
semiconducting materials.
In the case of the classical metals, the electrical contact 185
formed between the electrically-conductive structure 180 and the
semiconducting material inside the mesa 140 is typically a Schottky
contact. However, in the case of the semiconducting materials
mentioned above, the electrical contact behaves similar to a
Schottky electrical contact, but is not a Schottky contact 185 in
the classical sense. Therefore, an electrical contact 185, formed
between such a material of the electrically conductive structure
180 and the semiconducting material 140 is referred to as a
Schottky-like electrical contact 185. Also in this case, the
electrical contact is typically a unipolar electrical contact in
the forward-biased state as outlined before. In the reverse-biased
state, typically a blocking state exists as outlined before.
In the embodiment shown in FIG. 1, the electrically-conductive
structure 180 essentially fills the whole recess 120 and, hence,
forms the Schottky or Schottky-like electrical contact on the whole
sidewalls 190 of the recess 120 or the mesa 140. The
electrically-conductive structure 180 is further arranged on top of
a top surface 260 of the mesa 140 forming an upper part of the
Schottky or Schottky-like electrical contact with a semiconducting
material of the mesa 140. The electrically-conductive structure
arranged on the sidewall 190 of the mesa 140 forms a lower part of
the Schottky or Schottky-like electrical contact. As a consequence,
it may be possible to further reduce the threshold voltage in the
forward-biased state by enlarging the area covered by the
electrically-conductive structure 180.
However, optionally, the Schottky or Schottky-like electrical
contact 185 may be configured such that the upper part of the
Schottky or Schottky-like electrical contact comprises a lower
(forward) threshold voltage than the lower part of the Schottky or
Schottky-like electrical contact. This may, for instance, be
achieved by implementing different materials for the different
parts of the electrically-conductive structure 180. For instance,
the electrically-conductive structure 180 may comprise a top region
270 arranged on the top surface 260 of the mesa 140 and comprising
material which may have a lower Schottky barrier than a material
used for the lower part of the electrically-conductive structure
along the sidewalls 190. By arranging a material with a lower
Schottky barrier in the top region 270, it may be possible to
reduce the threshold voltage in the forward-biased state more
efficiently. By placing the material essentially only on the top
surface 260 and using a material with a higher Schottky barrier at
the sidewalls 190, may allow lower leakage currents in the
reverse-biased state by placing the material with a potentially
higher leakage current tendency further away from the doped regions
230 causing the pinching-off in the reverse-biased state.
However, optionally, to reduce the threshold voltage in the
forward-biased state further, it may be possible to implement the
mesa 140 comprising a height along the direction 150 into the
substrate being larger than a width perpendicular to the direction
150. In other words, the mesa 140 may comprise a height along the
direction 150 into the substrate 110 and a width perpendicular to
the direction 150 into the substrate 110 such that the height is at
least equal to the width. As a consequence, an area of the Schottky
or Schottky-like electrical contact 185 between the
electrically-conductive structure 180 and the semiconducting
material of the mesa 140 may become larger, which may lead to a
reduced threshold voltage. However, by increasing the distance to
the bottom plane 160 and, hence, to the optionally-implemented
doped regions 230, the risk of exceeding the characteristic reverse
voltage of the material used for the top region 270, when
implemented, may be also reduced, which may lead to an unacceptably
high leakage current.
In other embodiments, the height may be at least two times, at
least five times, at least ten times, at least twenty times or at
least fifty times larger than the width of the mesa 140. By
increasing the height compared to the width, the
previously-mentioned effects may eventually be amplified. However,
fabrication may also become more difficult leading to a higher
discard and, hence, to higher over-all costs for the semiconductor
device 100.
Optionally, the semiconductor device 100 may, hence, be configured
to cause a depletion of charge carriers in the area along the
projection 200 of the mesa 140 into the substrate 110 such that,
when a pinched-off voltage in the range of 5 V to 50 V is applied
to the electrically-conductive structure 180 and a counter
electrode 280 such that the Schottky or Schottky-like electrical
contact 185 is in the reverse-biased state, a characteristic
reverse electric field strength at the surface of the Schottky or
Schottky-like electrical contact 185 is not exceeded, which is also
referred to as a critical surface field strength, which is under
normal operating conditions not to be exceeded.
FIG. 2 shows a cross-sectional view of a semiconductor device 100
according to a further embodiment. The semiconductor device 100 of
FIG. 2 differs from the one shown in FIG. 1 by a few optional
modifications. For instance, the further doped region 250 extends
in the semiconductor device 100 shown in FIG. 2 further into the
substrate 110 than the doped regions 230 and below the doped
regions 230. When, for instance, the further doped region 250
comprising a doping concentration (third doping concentration)
being larger than both the first doping concentration of the mesa
140 and the second doping concentration along the projection 200
into the substrate 110, the further doped region 250 may be able to
distribute the current flowing between the electrically-conductive
structure 180 and the counter electrode 280. Due to the higher
doping concentration of the further doped region 250, an additional
voltage drop caused by the redistribution or spreading of the
current may be reduced compared to an implementation with a lower
doping concentration in the further doped region 250. In the case
the further doped region 250 extends further into the substrate 110
than the doped regions 230 and below the doped regions 230, the
further doped region 250 is also sometimes referred to as a current
spread region.
In other words, by implementing the further doped region 250 to
extend below the doped region 230 and extending further into the
substrate 110 than the doped region 230, the currents in the drift
area 210, which also referred to as drift zone, may be distributed
more evenly.
Depending on the materials used for the electrical conductive
structure 180 forming, for instance, the Schottky or Schottky-like
electrical contact 185 along the sidewalls 190, implementing an
electrical contact structure 290 may be an advisable option to
improve an electrical contact of the doped region 230 arranged
below and, therefore, along the projection 240 into the substrate
110 of the recess 120. The recess 120 may in this case comprise the
electrical contact structure 290 arranged on the bottom plane 160
of the recess 120. The electrical contact structure is configured
to electrically couple the doped region 230 through the
electrically-conductive structure 180.
The electrical contact structure 190 may, for instance, comprise a
material of a group of contact materials. The group of contact
materials may, for instance, comprise metals such as aluminum (Al),
titanium (Ti), copper (Cu), or nickel (Ni), alloys but also
(highly) doped poly-silicon (poly-Si), poly germanium (Ge) or the
like.
In the case an electric contact structure 290 is implemented, the
electrically-conductive structure 180 forming the Schottky or
Schottky-like electrical contact 185 may not extend along the full
sidewall 190 of the mesa 140 or the recess 120. In other words, the
semiconductor device 100 shown in FIG. 2 differs also from the
semiconductor device 100 of FIG. 1 by the electrically-conductive
structure 180 only partially extending along the sidewall 190 of
the recess 120 or the mesa 140. However, it should be noted that
implementing an electrical contact structure 290 is by far not
necessary to restrict the extension of the Schottky or
Schottky-like electrical contact 185 along the sidewall 190.
Independent of the question as to whether the electrical contact
structure 290 is implemented, an electrically-insulating structure
300 may be arranged partially along the sidewall 190 of the recess
120 at the bottom plane 160. By implementing the
electrically-insulating structure 300 it may be possible to limit
the extension of the Schottky or Schottky-like electrical contact
along the sidewall 190 and, independent of this aspect, to reduce
electrical field strengths at the corners of the recess 120 at the
bottom plane 160. By implementing the electrically-insulating
structure 300, it may therefore be possible to configure the
semiconductor device 100 to sustain higher voltages in the
reverse-biased state of the Schottky or Schottky-like electrical
contact 185.
The electrically-insulating structure 300 may comprise in principle
any insulating material such as silicon dioxide, aluminum oxide,
but also organic materials if applicable.
Due to the different approaches and optional features implemented
in a semiconductor device 100 according to an embodiment, the
semiconductor device 100 may be configured such that in the
reverse-biased state of the Schottky or Schottky-like electrical
contact 185, an electric field strength along the sidewall 190 of
the mesa 140 may be kept essentially constant along a portion of
the sidewall comprising at least 50% of a height along the
direction 150 into the substrate 110 of the sidewall 190.
Naturally, in other embodiments, increasing the
previously-mentioned ratio of 50% may be possible. For instance, it
may be possible for the portion to comprise at least 75%, at least
90% or even at least 95% of the height of the sidewall 190.
However, under some operation conditions or in other embodiments,
the portion may be smaller than the previously-mentioned 50%.
Optionally, the device may be designed such that the electric field
at the sidewall 190 at a normal operating voltage is at most 30% of
a maximum electric field below the bottom plane 160 and, hence, in
the bulk of the device 100.
Although in FIGS. 1 and 2 two recesses 120-1, 120-2 have been shown
forming the mesa 140, the number of recesses 120 is by far not
limited to two. In other embodiments, also a single recess 120
forming the mesa 140 may be implemented. However, in further
embodiments, more than one mesa 140 may be implemented as well
based on one or more recesses 120.
A semiconductor device 100 according to an embodiment may be a
discrete device or be part of a larger discrete device or an
integrated circuit. Examples come from different fields of circuit
elements as well as integrated circuits. As outlined before, a
semiconductor device 100 according to an embodiment may be
implemented as a stand-alone discrete device but may also be
integrated within an active switch or the like, for instance on the
basis of a silicon carbide substrate and semiconducting
material.
For instance, in the case of a stand-alone or discrete Schottky
diode implemented as a semiconductor device 100 according to an
embodiment, it may be possible to increase the area of the Schottky
or Schottky-like electric contact 185. In the case of implementing
a semiconductor device 100 according to an embodiment in the
framework of another device comprising an active switch or the
like, it may be possible to improve the forward characteristics of,
for instance, the body diode performance leading to lower costs
and, optionally, to a lower area consumption compared to more
conventional existing solutions. The body diode performance may,
for instance, be improved by decreasing the forward voltage drop
(threshold voltage) and/or a faster switching speed.
A semiconductor device 100 according to an embodiment may offer the
possibility of significantly increasing the area of the Schottky or
Schottky-like electrical contact 185. By this, it may be possible
to achieve a reduction of the effective threshold voltage in the
forward-biased state. The increase of the area may be implemented,
as described above, by narrow trench structures (trench 130, recess
120), which may be completely filled with the respective contact
material, such as a Schottky contact metal. The bottoms or floors
of the trenches 130 may be adjacent to p+-regions (doped region
230) which may shield electrical fields in the reverse-biased state
comparable to a MPS-structure (merged PIN Schottky diode). As a
consequence, it may be possible to have comparably small electrical
fields at the sidewalls 190 forming the Schottky interfaces between
the electrically-conductive structure 180 and the semiconducting
material of the mesa 140.
It may be possible to implement the doped regions 230 and the lower
doped n-doped region between the doped regions 230 such that they
form a JFET-like structure (Junction Field Effect Transistor)
having a pinched-off voltage of approximately 5 V to 10 V, which is
typically smaller than the pinched-off voltage in the mesa 140. As
a consequence, it may be possible to limit the voltages present at
the Schottky or Schottky-like electrical contact to this value and
to limit the leakage current in the reverse-biased state
accordingly.
By increasing the area of the Schottky or Schottky-like electric
contact by a factor of 10 compared to a planar implementation, the
threshold voltage may eventually be reduced by approximately 0.1 V.
Increasing the area further, may lead to a further reduction.
However, to take full advantage of the specific area of the
Schottky or Schottky-like contact, it may be advisable to increase
the doping concentration of a semiconducting material in the mesa
140 by approximately one or two orders of magnitude to reduce a
resistance inside the mesa 140 for the charge carriers. The higher
the doping concentration, the more evenly distributed is the
current density along the sidewalls 190 of the mesa 140.
Additionally, the higher doping of the mesa 140 may also lead to a
further reduction of the threshold voltage by approximately 0.1
V.
Due to the comparably small threshold voltages in the
forward-biased state and the characteristic reverse voltages in the
reverse-biased state and the resulting comparably small leakage
currents, it may be possible to use a Schottky metallization or
another Schottky-like material with a small barrier height such as
titanium (Ti). However, as outlined before, alternatively or
additionally, also p+- or n+-poly silicon (poly Si) may be used. As
a consequence, it may be possible to reduce the threshold voltage
by approximately 0.5 V compared to more conventional solutions.
By implementing a semiconductor device 100 according to an
embodiment, it may be possible to increase the area of the Schottky
or Schottky-like electrical contact without significantly changing
the device's footprint and the leakage current.
In the following, with reference to FIGS. 3, 4 and 5, a silicon
carbide trench Schottky diode will be described in more detail as
an embodiment of a semiconductor device 100. FIG. 3 shows a
cross-sectional view of a semiconductor device 100 in the form of a
silicon carbide Schottky diode 310. The silicon carbide Schottky
diode 310 comprises a plurality of trenches 130 forming the
recesses 120 having the previously-described bottom plane 160. The
recesses 120 are filled with a Schottky metal to form the
electrically-conductive structure 180 forming in turn the Schottky
electrical contacts 185 along the sidewalls 190 and at the top
surface 260 of the mesas 140. However, it should be noted that once
again instead of the Schottky metal to fill the recesses 120,
p-doped poly-silicon may also be used as well as other materials.
Additionally, on top of the mesa 140 at its top surface 260 a
Schottky metal or another p-doped poly-silicon may be used. In the
case of poly-silicon being used to form at least partially the
electrically-conductive structure 180, the electrically-conductive
structure 180 forms at least partially a Schottky-like electrical
contact 185 with a semiconducting material of the mesa 140. The
semiconducting material in the mesa a highly n-doped silicon
carbide (n+) filling the mesa 140 down to the bottom plane 160.
Below the bottom plane 160 along the direction 150, the substrate
110 comprises in the drift area 210 an n-doped silicon carbide
semiconducting material (n) and at the back side 220 a counter
electrode 280 formed by a highly n-doped silicon carbide
semiconducting material (n+). The counter electrode 280 is coupled
to a terminal 320 forming the anode of the Schottky diode 310.
Correspondingly, a terminal 330 is electrically coupled to the
electrically-conductive structure 180, the terminal 330 and the
electrically-conductive structure 180 forming the cathode of the
Schottky diode.
Below the recesses 120 doped regions 230 are implemented along the
direction 150 into the substrate 110. In other words, the doped
regions 230 are arranged along a projection 200 of the recesses 120
into the substrate 110. The doped regions 230 comprise highly
p-doped silicon carbide semiconducting material (p+).
FIG. 3 shows, therefore, a trench-Schottky diode 310 with a large
contact area between the electrically-conductive structure 180 and
the semiconducting material of the mesas 140 forming the Schottky
or Schottky-like electrical contacts. Such a device may, for
instance, be fabricated using the following process operations. In
an early process operation, via a resist mask used for a junction
termination extensions (JTE), the doped regions 230 may be
implanted and depositing an epitactical layer of the semiconducting
material afterwards.
However, the doped regions 230 may also be fabricated in a
self-aligned way by doping the semiconducting material at a later
state in the process.
Afterwards, a hard mask for etching the trenches can be deposited
and optionally, solidified and patterned by lithographic operations
and dry etching processes. The lithographic operations may comprise
deep-UV lithography (UV=Ultra Violet). Afterwards, the trenches can
be etched. A high temperature tempering process to round edges at
the bottom of the trenches 130 may follow. After lifting off the
hard mask, the trenches may be filled, for instance, with
poly-silicon or nickel-aluminum (NiAl) following a planarization
operation. The poly-silicon can also be used on the top surface 260
of the mesa 140 as an electrical contact. In this case, it may be
advisable not to etch back down to the top surface 260 of the
silicon carbide mesa 140. Optional temper processes may also be
used here.
Afterwards, the front side and the back side 220 may be processed,
for instance, including depositing a protective layer (e.g. an
imide), applying a front side metallization (FSM) and a back side
metallization (BSM).
However, the doped regions 230 may also be formed in a self-aligned
way. Before lifting of the hard mask for etching the trenches 130,
a p-implantation of the bottom of the trenches 130 (recesses 120)
may be done using the hard mask.
To protect the mesa 140 and as an additional protection for the
sidewalls 190, a thermal oxidation may be carried out. Depending on
the semiconductor material used, an orientation of the crystalline
structure may exhibit substantially different oxidation rates. As a
consequence, a thicker oxide may stop the ions of the implantation
from penetrating the sidewalls 190 since their angle of incidence
with respect to the surface of the sidewalls 190 is very shallow.
However, at the bottom of the trenches 130, where the angle of
incidence is approximately 90.degree., the ions can penetrate a
thinner oxide layer and create the desired p-implantation. After
the implantation, the thermal oxide can be lifted off along with
the hard mask. In the case of a thermal oxidation, the oxide may
grow on the sidewalls 190 up to several times (e.g. five times)
faster than on the bottom of the trench 130, supporting the process
described above.
Optionally, the trenches 130 or recesses 120 may be formed such
that sidewalls 190 are parallel to a (112;.sup.- 0)-plane (=11-20)
or a (11;.sup.- 00)-plane (=1-100). The main surface of the die at
it top side may, for instance, be parallel to the (0001)-plane,
while its backside may be parallel to the (0001;.sup.-)-plane
(=000-1). This may be fabricated by aligning the trenches 130 such
that their sidewalls first are approximately parallel to the
desired plane. Then, by tempering the sample in a hydrogen
(H.sub.2) atmosphere, the precise planes will form.
The Schottky diode 310 shown in FIG. 3 is based on the idea of
separating electrically and spatially the Schottky diode and the
drift area 210. This is achieved by enlarging the contact area of
the Schottky or Schottky-like electrical contact per unit chip area
by a factor of one or two orders of magnitude (factor of
approximately 10 to approximately 100) which may lead to a
reduction of a threshold voltage of approximately 0.1 to 0.2 V.
Moreover, the doping concentration of the semiconductor material in
the mesa 140 is increased by one or two orders of magnitude
compared to a doping concentration in the drift area 210, which
may, for instance, comprise the second doping concentration as laid
out before. This may also lead to a reduction of the threshold
voltage by approximately 0.1 to 0.2 V.
By clearing the mesa 140 at comparably small voltages beginning in
the range of approximately 5 to 10 V, the electrical field strength
at the Schottky or Schottky-like electrical contact may be kept
small. Accordingly, the leakage current may also be kept small.
Technically, this is done by the doped regions 230 (p+-regions),
which clear or deplete the semiconducting material at the bottom
level 160 of the mesas 140 (n-regions) at smaller voltages. As a
consequence, the Schottky or Schottky-like electrical contacts may
be shielded.
As a consequence, it may be possible to use a material for the
electrically-conductive structure 180 such as a Schottky
metallization having a smaller work function like, for instance,
titanium (Ti) or hafnium (Hf). As outlined before, an n+-doped
poly-silicon or an n-doped silicon carbide material for the
electrically-conductive structure 180 may also be used.
Optionally, fabricating the nickel-aluminum contacts may be done
prior to depositing the poly-silicon, when the poly-silicon is used
as a Schottky-like electrical contact on the front side of the die.
The nickel-aluminum-regions may then be electrically contacted by
the poly-silicon.
Similar to the processes described above, it may also be possible
to deposit on top of the trenches 130 for instance an n-doped
poly-silicon with a lower barrier or another material such as
molybdenum (Mo) or hafnium (Hf) having a Schottky barrier with
respect to silicon carbide (SiC) of not more than 1.1 eV. When on
the top surface 260 of the mesas 140 a different material than
poly-silicon is used as a Schottky-like material, additionally the
barrier of the semiconducting material at the top surface 260 of
the mesa 140 may be reduced by a shallow n-implantation.
Alternatively, a Schottky material such as hafnium (Hf), molybdenum
nitride (MoN) or titanium nitride (TiN) or another material with a
lower work function with respect to a silicon carbide may be
used.
Moreover, it may be possible to configure the width of the mesa 140
and its doping concentration (e.g. the first doping concentration)
such that the mesa 140 is depleted at a reverse voltage of
approximately 10 V. In this case, the upper part of the
electrically-conductive structure forming a Schottky or
Schottky-like electrical contact is then only required to have a
characteristic reverse voltage of approximately 10 V. Naturally,
instead of 10 V, which merely represents one example, the mesa 140
and the material(s) used for the electrically-conductive structure
180 may be configured in such a way that any other
technically-feasible voltage level as described above in terms of
the example of 10 V may be used.
Moreover, before the above-mentioned process operations for
fabricating a Schottky diode 310, a silicon carbide layer may be
epitactically grown, which comprises a higher doping concentration
than the drift area 210. This layer is under ideal circumstances
approximately as thick as the trenches 130 or the recesses 120
formed in the following process operations. As a consequence, the
drift area 210 may start at the bottom level 160 of the recesses
120 or trenches 130. In combination with a p-implantation at the
bottoms of the trenches 130 or recesses 120 it may be possible to
deplete the mesas 140 at very low voltages to enable the Schottky
or Schottky-like contacts to be shielded off at higher electrical
field strength.
Moreover, lower parts of the sidewalls of the mesas 140 or of the
recesses 120 may be p-implanted to accelerate the forming of the
depletion region or space-charge region in the trenches 130 or the
recesses 120.
Moreover, the mesas 140 may be formed having a conical
cross-section, for instance, being narrower at the bottom level 160
than at the top surface 260. In this case, the doping concentration
inside the mesa 140 may be configured such that the mesa may be
homogeneously depleted. This may be achieved by adjusting the
doping concentration levels inside the mesa 140 in such a way that
the doping concentration is lower where the mesa 140 is wider.
Using an appropriate line of process of fabricating the hard mask
for etching the trenches 130 and the following etching of the
trenches, an angle of the sidewalls 190 larger than 90.degree. at
the bottom level 160 may be realized. In other words, the mesas 140
may be formed conically reducing their widths above the bottom
level towards the top surface 260. By using an n-implantation of
the mesas 140 or by adapting the doping concentration of the
silicon carbide layer as a function of its thickness as outlined
above, the doping level may be adapted of the mesas 140
accordingly. As a consequence, it may be possible to pinch off the
mesas 140 along their full height at the same time.
The doping of the mesas 140 may also be carried out by a sidewall
implantation instead of using epitactical growth. For instance, by
implanting the semiconductor material of the mesas 140 at an angle
being different from 0.degree. with respect to the direction 150,
the sidewalls 190 of the mesas 140 may be implanted or doped. Due
to the presence of mesas 140 in the vicinity, a shadowing of
neighboring mesas may occur. As a consequence, a part of the mesa
140 at the bottom level may eventually not be implanted or doped.
As a consequence, it may be possible to implement different barrier
heights for the Schottky or Schottky-like electrical contacts along
the height of the mesa 140 using, for instance, an n-implantation.
By using several implantation operations at different angles, a
number of regions with different barrier heights may be realized.
Using N implantation processes, may, therefore, lead to (N+1)
different regions. It may therefore be possible, to control the
pinching-off of the mesas 140 more closely, when, for instance, not
only the dose and the angle of the implantation, but also the
energy used is varied, for instance in the framework of a
p-implantation.
As outlined before, the barrier at the top surface 260 of the mesa
140 may be reduced by doping or implantation. Here, the same
material for the Schottky barrier may be used on the top surface
260 and along the sidewalls 190 in the recess 120 or the trench
130.
Further process variations comprise, for instance, the
previously-mentioned thermal oxidation of the sidewalls 190, which
may be used to create a thicker oxide to protect the sidewalls 190
during the trench bottom implantation, as outlined above.
In other words, FIG. 3 shows a schematic representation of a
three-dimensional Schottky diode, which offers the possibility of
increasing the area of the Schottky or Schottky-like electrical
contact independent of a width of the Schottky region. This may
allow decreasing the costs and the form factor of these devices.
Moreover, it may be possible to increase the shielding of the
Schottky diode in a blocking mode of operation. Using an embodiment
may therefore allow a monolithic integration of a three-dimensional
silicon carbide Schottky diode 310 within a silicon carbide active
switch or a similar structure. The third dimension or --in other
words--the sidewalls 190 may be used to increase the area for the
Schottky or Schottky-like electrical contact.
FIG. 4 shows a cross-sectional view of a basis for a simulation of
the electrical behavior of a semiconductor device 100 according to
an embodiment. To be more precise, FIG. 4 shows a recess 120 in the
form of a trench 130 and a mesa 140. The recess 120 comprises an
electrically-conductive structure 180, which forms a Schottky or
Schottky-like electrical contact 185 with the semiconducting
material comprised in the mesa 140. The Schottky or Schottky-like
electrical contact 185 does not extend all the way down to the
bottom level 160 of the recess 120. The recess 120 may, for
instance, comprise at a level 350 indicated by a dashed line in
FIG. 4 an electrically-insulating structure 300 at the sidewall 190
of the mesa 140 or the recess 120, which is, however, not shown in
FIG. 4.
The bottom level 160 further indicates a boundary of a layer of the
semiconducting material used for the mesa 140. Inside the mesa 140
the semiconducting material (e.g. silicon carbide, SiC) comprises
the first doping concentration. Beginning at the bottom plane 160
and extending further into the substrate 110 along the direction
150, a further doped region 250 is implemented having a third
doping concentration being higher than the first doping
concentration in the mesa 140. The further doped region 250 extends
further into the substrate 110 along the direction 150 than a doped
region 230 having the opposite conductivity type than the
semiconducting material in the mesa 140, the further doped region
250 and a drift area 210 following the further doped region 250
beginning at a level 360. The doping concentration of the further
doped region 250 (third doping concentration) is also higher than a
doping concentration of the semiconducting material of the drift
area 210 (second doping concentration). As a consequence, the
further doped region 250 acts once again as a current spread
structure enabling a more evenly distributed current flow in the
lower doped drift area 210. This may reduce a voltage drop across
the drift area 210 in the forward-biased state of operation of the
semiconductor device 100.
The doped region 230 having the opposite conductivity type than the
semiconducting material of the mesa 140, the further doped region
250 and the drift area 210 extends into a projection of the mesa
140 below the bottom plane 160. As a consequence, it may be
possible to pinch off the mesa 140 in the reverse-biased state more
easily.
FIG. 5 shows a result of a numerical simulation with respect to the
absolute values of the electrical fields inside the mesa 140 and
the further components of the semiconductor device 100 shown in
FIG. 4 at a voltage of 650 V applied in the reverse-biased state.
FIG. 5 illustrates that in the reverse-biased state the electrical
fields along the sidewalls 190 of the recess 120 or the trench 130
are essentially constant. It may therefore be possible to use a
material with a lower Schottky or Schottky-like barrier along the
sidewalls 190 of the recess 120.
In the following further examples of an implementation of a
Schottky or Schottky-like electrical contact for a junction field
effect transistor (JFET) and a vertical field effect transistor
(MOSFET=Metal-Oxide-Semiconductor Field Effect Transistor) will be
shown. These implementations as well as the Schottky diode 310
shown above may be implemented based on silicon carbide (SiC), but
may also be implemented based on different semiconducting materials
and substrates.
FIG. 6 shows a cross-sectional view of a further semiconductor
device 100 according to an embodiment in the form of a junction
field effect transistor 400 (JFET) arranged at least partially on a
substrate 110. The JFET 400 comprises a recess 120, which may, for
instance, be fabricated as a trench. The recess 120 forms a mesa
140 extending along a direction 150 into the substrate 110 to a
bottom plane 160 of the recess 120. The mesa 140 comprises a
semiconducting material of a first conductivity type, for instance
an n-doped semiconducting material. The semiconducting material may
be in principle any semiconducting material, although in the
following the description will be focused on silicon carbide (SiC).
This, however, only represents one example for a possible
semiconductor material.
The JFET 400 further comprises an electrically-conductive structure
180 arranged at least partially along a sidewall 190 of the mesa
140. The electrically-conductive structure 180 forms a Schottky or
Schottky-like electrical contact 185 with the semiconducting
material of the mesa 140 as outlined before.
The JFET further comprises a doped region 230 of a second
conductivity type arranged at least partially adjacent to the
bottom plane 160 of the recess 120 along a projection into the
substrate 110 following the direction 150 such that the mesa 140 is
accessible by charge carriers avoiding the doped region 230. In the
embodiment shown in FIG. 6, the doped region 230 is formed by a
highly p-doped silicon carbide semiconducting material (p+). The
doped region 230 in the example shown here does not extend into a
projection of the mesa 140 into the substrate 110 along the
direction 150. This, however, may be the case in other
embodiments.
The JFET further comprises a drain contact 410 formed as a metal
contact on a counter electrode 280 formed by a highly n-doped
semiconducting material which may be optionally provided as part of
the substrate. On top of the counter electrode 280, a drift area
210 may be implemented, which may be part of the substrate 110 or
may be an epitactical layer grown on the substrate.
The drift area 210 comprises the semiconducting material of the
first conductivity type. In the embodiment shown in FIG. 6, the
drift area 210 is a lowly n-doped semiconducting material (n-). The
drift area 210 is arranged along the direction 150 into the
substrate 110 between the drain contact 410 and the
electrically-conductive structure 180.
The JFET 100 further comprises a source region 420 coupled to the
electrically-conductive structure 180 and comprising the
semiconducting material of the first conductivity type. Here, it is
a highly n-doped semiconducting material. The source region 420 is
shut off from the drift area 210 by a semiconductor region 430 of a
second conductivity type, which is at least partially formed by the
doped region 230. The semiconducting region 430 is also referred to
as the body of the JFET 400.
In the embodiment shown in FIG. 6, the semiconducting region 430
shutting off the source region 420 from the drift area 210 is
formed by highly p-doped semiconducting material (p+), which may be
identical with the doped region 230 or comprise the doped region
230. Naturally, the semiconducting region 430 may also comprise
different doping levels. In this case, the doped region 230 may
eventually comprise a different doping concentration.
The JFET 400 further comprises a gate stack arrangement 440
comprising a first layer of the first conductivity type, a second
layer 460 of the second conductivity type and a gate contact 470.
The second layer 460, which is implemented here as a highly p-doped
layer, while the first layer 450 is implemented as a lowly n-doped
layer (n-), is arranged between the first layer 450 and the gate
contact 470. However, in other embodiments the doping concentration
and the thickness may be chosen such that the desired pinch-off
voltage is implemented. The first layer 450 is in electrical
contact with the source region 420 and the source region 430 of the
second conductivity type.
The mesa 140 along with the electrically-conductive structure 180
forms a Schottky diode or Schottky-like diode 310 which may
optionally comprise the doping concentrations as outlined above in
the context of FIGS. 1 to 5.
In operation, a channel may form in the first layer 450 of the gate
stack arrangement 440, which is controllable by a voltage applied
to the gate contact 470. Depending on the control voltage applied
to the gate contact 470, the channel forming in the first layer 450
may be controlled by creating a depletion zone at the pn-junction
between the first and second layers 450, 460. The channel may even
be fully pinched off when applying an appropriate voltage to the
gate contact 470.
In other words, a three-dimensional Schottky or Schottky-like
contact 184 may be integrated within a body diode contact of a
semiconductor device 100 in the form of a JFET 400 as shown in FIG.
6. It is to be noted that FIG. 6 shows a more schematic view of a
JFET 400 according to an embodiment. When implementing the device
based on silicon carbide, the JFET 400 is also referred to as a SiC
JFET. The electrically-conductive structure 180 may form the
electrical contact for the source region 430 and the body diode
formed here as a Schottky diode 310. It may, as outlined before, be
formed by a metal contact, but may also comprise other materials
mentioned above. In yet other words, FIG. 6 shows a schematic
representation of a silicon carbide JFET with a monolithic
integration of a three-dimensional silicon carbide Schottky diode
310.
FIG. 7 shows a schematic cross-sectional view of a SiC trench
MOSFET with a monolithic integration of a three-dimensional silicon
carbide Schottky diode 310. To put it in different terms, FIG. 7
shows another semiconductor device 100 according to an embodiment
in the form of a vertical field effect transistor (FET) 500. The
FET comprises once again a recess 120 forming a mesa 140 which
extends along a direction 150 into the substrate 110 to a bottom
plane 160 of the recess 120. The mesa 140 comprises once again a
semiconducting material of a first conductivity type.
The recess 120 may once again be optionally formed by a trench but
may also be formed by growing an epitactical layer on a surface of
a substrate 110 and creating the recess 120 forming the mesa 140.
In the embodiment depicted in FIG. 7 the second approach has been
used.
The FET 500 according to an embodiment further comprises an
electrically-conductive structure 180 arranged at least partially
along a sidewall 190 of the mesa 140 as outlined before. The
electrically-conductive structure 180 forms a Schottky or
Schottky-like electrical contact 185 with the semiconducting
material of the mesa 140. Once again, the semiconductor device 100
in the form a FET 500 comprises a doped region 230 of a second
conductivity type (highly p-doped semiconducting material in the
embodiment shown in FIG. 7) which is at least partially arranged
adjacent to the bottom plane 160 of the recess 120 along a
projection into the substrate 110 along the direction 150 such that
the mesa 140 is accessible by charge carriers avoiding the doped
region 230. Here, one again, the doped regions 230 do not extend
into the projection of the mesa 140 into the substrate 110.
However, in other embodiments, the doped regions 230 may extend
into the previously-mentioned projection of the mesa 140.
Similar to the drain contact 410, the FET 500 also comprises at a
back side 220 of the substrate 110 a drain contact 510 which may be
formed by a metal to name just one alternative. The drain contact
510 is once again arranged on the counter electrode 280, which is
once again formed by a highly n-doped semiconducting material
(n+).
The FET 500 further comprises a drift area 210 comprising the
semiconducting material of the first conductivity type. In the
example shown in FIG. 7, it is implemented as a lowly n-doped
layer. The drift area 210 is arranged along the direction 150 into
the substrate 110 between the drain contact 510 and the
electrically-conductive structure 180.
The FET 500 further comprises a source region 520 electrically
coupled to the electrically-conductive structure 185. It comprises
the semiconducting material of the first conductivity type. Here,
it is implemented as a highly n-doped region. The FET 500 further
comprises a body region 530 comprising the semiconducting material
of the second conductivity type. The body region 530, which is also
referred to as body only, is implemented here as a lowly p-doped
region. It is arranged along the direction 150 into the substrate
110 between the source region 520 and the drift area 210.
Furthermore, the FET 500 comprises a gate contact 540 arranged in a
trench 550 extending into the substrate 110. The gate contact 540
is electrically insulated from the source region 520, the body
region 530 and the drift area 210 by an insulating film 560
covering at least partially a sidewall 570 and a bottom 580 of the
trench 550. The source region 520 is arranged in a direction
perpendicular to the direction 150 into the substrate 110 between
the trench 550 and the doped region 230. The channel region 530 is
at least partially arranged in the direction perpendicular to the
direction 150 into the substrate 110 between the trench 550 and the
doped region 230.
Once again, the mesa 140 along with the electrically-conductive
material 180 forms the Schottky diode 310. Therefore, FIG. 7 shows
a SiC trench MOSFET 500 with a monolithic integration of a
three-dimensional SiC Schottky diode 310. As outlined in context
with FIG. 6, the electrically-conductive structure 185 may be
implemented as a metal contact for the source and the body
diode.
Also in the case of the FET 500 according to an embodiment, the
previously-described doping concentrations concerning the Schottky
diode 310 may be implemented as outlined in the context of FIGS. 1
to 5.
Devices may comprise a plurality of any of the above-mentioned and
described structures and circuit elements, which may be coupled in
parallel, depending on the desired currents and other parameters.
For instance, such a device may comprise a plurality of trenches
130, recesses 120 and mesas 140, which may be coupled in parallel.
However, also a series connection or more complex connections may
be implemented. Moreover, such a device may comprise a termination
(e.g. a JTE; junction termination extension).
The description and drawings merely illustrate the principles of
embodiments. It will thus be appreciated that those skilled in the
art will be able to devise various arrangements that, although not
explicitly described or shown herein, embody the principles of the
invention and are included within its spirit and scope.
Furthermore, all examples recited herein are principally intended
expressly to be only for pedagogical purposes to aid the reader in
understanding the principles of the invention and the concepts
contributed by the inventor(s) to furthering the art, and are to be
construed as being without limitation to such specifically recited
examples and conditions. Moreover, all statements herein reciting
principles, aspects, and embodiments of the invention, as well as
specific examples thereof, are intended to encompass equivalents
thereof.
Functional blocks denoted as "means for . . . " (performing a
certain function) shall be understood as functional blocks
comprising circuitry that is adapted for performing or to perform a
certain function, respectively. Hence, a "means for s.th." may as
well be understood as a "means being adapted or suited for s.th.".
A means being adapted for performing a certain function does,
hence, not imply that such means necessarily is performing said
function (at a given time instant).
The methods, for instance fabrication processes, described herein
may be implemented as software or with the help of software, for
instance, as a computer program. The sub-processes may be performed
by such a program by, for instance, writing into a memory location.
Similarly, reading or receiving data may be performed by reading
from the same or another memory location. A memory location may be
a register or another memory of an appropriate hardware. The
functions of the various elements shown in the Figures, including
any functional blocks labeled as "means", "means for forming",
"means for determining" etc., may be provided through the use of
dedicated hardware, such as "a former", "a determiner", etc. as
well as hardware capable of executing software in association with
appropriate software. When provided by a processor, the functions
may be provided by a single dedicated processor, by a single shared
processor, or by a plurality of individual processors, some of
which may be shared. Moreover, explicit use of the term "processor"
or "controller" should not be construed to refer exclusively to
hardware capable of executing software, and may implicitly include,
without limitation, digital signal processor (DSP) hardware,
network processor, application specific integrated circuit (ASIC),
field programmable gate array (FPGA), read only memory (ROM) for
storing software, random access memory (RAM), and non-volatile
storage. Other hardware, conventional and/or custom, may also be
included. Similarly, any switches shown in the Figures are
conceptual only. Their function may be carried out through the
operation of program logic, through dedicated logic, through the
interaction of program control and dedicated logic, the particular
technique being selectable by the implementer as more specifically
understood from the context.
Furthermore, the following claims are hereby incorporated into the
Detailed Description, where each claim may stand on its own as a
separate embodiment. While each claim may stand on its own as a
separate embodiment, it is to be noted that--although a dependent
claim may refer in the claims to a specific combination with one or
more other claims--other embodiments may also include a combination
of the dependent claim with the subject matter of each other
dependent claim. Such combinations are proposed herein unless it is
stated that a specific combination is not intended. Furthermore, it
is intended to include also features of a claim to any other
independent claim even if this claim is not directly made dependent
to the independent claim.
It is further to be noted that methods disclosed in the
specification or in the claims may be implemented by a device
having means for performing each of the respective steps of these
methods.
Further, it is to be understood that the disclosure of multiple
steps or functions disclosed in the specification or claims may not
be construed as to be within the specific order. Therefore, the
disclosure of multiple steps or functions will not limit these to a
particular order unless such steps or functions are not
interchangeable for technical reasons.
Furthermore, in some embodiments a single step may include or may
be broken into multiple substeps. Such substeps may be included and
part of the disclosure of this single step unless explicitly
excluded.
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